Health Issues at a Glance

Science has learned more about marijuana in the past 20 years than in the preceding two centuries. Ironically, however, there has been a major disconnection between the scientific knowledge gained and the public’s understanding of the drug.

People often refer to their own experiences with marijuana, rather than to what science has taught us. No matter what people think about the drug and the policies surrounding it, it is vitally important to be well-versed in the science of marijuana use and addiction.

Sifting through the rhetoric can be difficult, but we now have a plethora of scientific studies from which to draw firm conclusions about marijuana use and its public-health implications. We provide a general summary for you here.

Marijuana and The Brain

Marijuana use directly affects the brain — specifically the parts of the brain responsible for memory, learning, attention and reaction time. These effects can last up to 28 days after abstinence from use.[1]

Science confirms that the adolescent brain — particularly the part of the brain that regulates planning for complex cognitive behavior, personality expression, decision making and social behavior — is not fully developed until the early to mid-20s. Developing brains are especially susceptible to all of the negative effects of marijuana and other drug use.[2]

What makes marijuana harmful? Three simple letters: T-H-C

Today’s marijuana is not your mama’s Woodstock weed.

Marijuana contains about 500 components, most of which we know little about.

The most prominent component is called THC. Scientists have found that THC is what produces the “high” users experience. In today’s street marijuana, which is usually smoked, producers have increased THC levels more than fourfold[3], and they have reduced the natural levels of other components that actually have been shown to reduce a user’s high.

Higher THC content can increase all of the drug’s usual negative effects.[4], [5] For example, since 1990, more people in the U.S. have gone to the emergency room after using marijuana even though overall numbers of marijuana users have remained relatively stable.[6],[7]

The main health harms of marijuana can be summarized as follows:

Heart: Marijuana use can cause an increase in the risk of a heart attack more than fourfold in the hour after use, and it can provoke chest pain in patients with heart disease. [8]

Lungs: Research has shown marijuana smoke contains carcinogens that can be irritants to the lungs, resulting in greater prevalence of bronchitis, cough and phlegm production.[9] Marijuana smoke contains 50-70 percent more carcinogenic hydrocarbons than does tobacco smoke, as reported by the American Lung Association.[10] Scientists have not found a definitive link between lung cancer and marijuana use.

Mental Health: Marijuana use is significantly linked with mental illness, especially schizophrenia and psychosis, but also depression and anxiety.[11]

Pregnancy: Marijuana smoking during pregnancy has been shown to decrease birthweight, most likely due to the effects of carbon monoxide on the developing fetus.[12]

Marijuana and Addiction

Is marijuana addictive?

Yes — especially today’s marijuana, which is between four and five times stronger than the weed of the 1960s and 1970s. Research has found that 1 in 11 of all marijuana users will become addicted to the drug. And if a person begins using under the age of 18, that number rises to one in six people. [13] We also know that almost 60 percent of new marijuana users each year are under age 18. Marijuana is the No. 1 reason adolescents are admitted to substance-abuse treatment in the U.S.

None of this means we need to lock up people for using marijuana. But it does mean we should be honest about the drug’s real dangers. And legalization — with all of the American-style promotion that will accompany it — is the last thing people in recovery, parents, communities — and even our nation — need right now.Think about it.

Data from the National Institute on Drug Abuse found that in 1993, marijuana comprised approximately 8 percent of all treatment admissions, but by 2009 that number had increased to 18 percent.[14] For those under age 18, marijuana-related treatment admissions increased by 188 percent from 1992 to 2006, while admissions for use of other drugs remained steady.[15]

Data in the United States are corroborated with data from other countries. In the European Union, the percentage of marijuana as the primary reason for entering treatment increased by 200 percent from 1999 to 2006 and currently stands at around 30 percent of all admissions.[16] The Netherlands has the highest rate of marijuana addiction in Europe. [17]

Marijuana and Driving

In the past decade, researchers from all corners of the world have documented the problem of marijuana use and driving.[18],[19],[20],[21],[22],[23]Linked to deficits in the parts of the brain that are important for driving, including the impairment of motor coordination and reaction time, a widely-cited article in a 2012 edition of the British Medical Journal concluded that marijuana use doubles the risk of car crashes.[24] Another recent meta-analysis of nine studies found that marijuana “… use by drivers is associated with a significantly increased risk of being involved in motor vehicle crashes.”[25]

Marijuana use and performance at school and on the job

One of the most well designed studies on marijuana and intelligence, released in 2012, found that marijuana use reduces IQ by as much as eight points by age 38 among people who started using marijuana regularly before age 18. Even people who had stopped using the drug before their 38th birthday experienced the loss in IQ.[26] Other studies have found that marijuana use is linked with school dropout and subsequent unemployment, social welfare dependence and a self-reported lower quality of life than non-marijuana-abusing people.[27]

According to the U.S. National Survey on Drug Use and Health, youth with poor academic results were more than four times as likely to have used marijuana in the past year than youth with an average of higher grades. This is consistent with an exhaustive meta-analysis examining four dozen different studies by Macleod and colleagues, published by Lancet. Those researchers found that marijuana use is consistently associated with lower grades and a decreased chance of graduating from school.[28] Ellickson and colleagues at the RAND Corporation surveyed almost 6,000 students ages 13 to 23 and found that the teens who smoked cannabis from once a week to monthly at age 13 and decreased their use by age 18, and decreased their use again to 3-10 times a year in young adulthood, still lagged behind all other groups in earnings and education when resurveyed at age 29.[29]

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